If those are your four cases I would go into the Properties for that layer and under Style select Interval, set it to 4 classes, set the values to those you described, and then set the style manually for each class.
Not as elegant as connecting it directly to a data column, but I've never really gotten that to work well without creating a new column for that specific purpose (which is also a bit inelegant.)
Edit: It turns out the word is 'Graduated' rather than 'Interval.' I have a translated version of QGIS and made the wrong assumption about the translation.